How to persist an optionsGroup with multiselect

Firstly, thank for a great platform!

I am unsure on the best way to persist an optionsGroup with multiselect enabled. In the example attached I have a simple phone messaging app where I want the message taker to be able to select e.g. Urgent and Personal.

I realise that the values returned from the optionsGroup component is a Collection, but I could not manage to either convert the collection to a delimited string (and back), or to persist the options selected to a related entity (composition).

Any working example / guidance will be appreciated.
multitest.zip (82.9 KB)